It is now well accepted that the gastrointestinal tract actively participates in foreign substance metabolism. Recently, similar activities have been demonstrated also in pulmonary tissue. Further exploration of these functions are aimed. Steroids are used as model substrates in part of these studies. Further identification of the biochemical machinery carrying out these numerous functions are attempted. Th adaptation of these functions on subcellular level under various experimental conditions are to be studied. The studies include whole organ perfusion, slice, cellular subfraction and tissue culture techniques combined with spectrometric and chromatographic procedures and autoradiographic methods.